The amino acid responses of neurons in the cat spinal trigeminal nucleus were studied by iontophoresis using multi-barrelled micropipettes in twenty cats, two to four weeks following unilateral retrogasserian (rhizotomy). A population of deafferented spontaneously hyperactive neurons (Type 2) was identified with grossly decreased responses to exogenous amino acids. The amino acid hyposensitivity was nonspecific for both inhibitory (GABA, Glycine or Alanine) or excitatory (Homocysteic acid, Glutamate or Aspartic acid) agents, except in a small subgroup (Type 2b) which was specifically hyposensitive to GABA. Silent (Type 1) or bursting (Type 3) neurons were found to have normal amino acid response compared to the intact spinal trigeminal nucleus. The results provided no support for Cannon's law of denervation supersensitivity in the central nervous system. Amino acid hyposensitivity was proposed as a physiological property of central neurons undergoing transneural atrophy.